1. Field of the Invention
The present invention relates to a pick-up roll apparatus for picking up a tail of a web from a forming wire. More particularly, the present invention relates to a pick-up roll apparatus which transfers the tail to a press felt which wraps around a portion of the pick-up roll apparatus.
2. Information Disclosure Statement
In the papermaking art, a web is formed on a fourdrinier wire by ejecting stock onto the wire and draining water from the stock through the wire.
Subsequently, the partially dewatered web is transferred from the fourdrinier forming wire onto a press felt where the web is conveyed between counter-rotating press rolls, or the like, for further dewatering the web.
Typically, the transfer from the forming wire to a press felt is accomplished by means of a pick-up roll, which is disposed closely adjacent to the forming wire. The press felt wraps around a portion of the pick-up roll such that the formed web is disposed between the fourdrinier wire and the press felt.
In many pick-up rolls, a central core of the pick-up roll defines a full-width chamber having an open face towards that portion of the pick-up roll wrapped by the press felt. The chamber which extends in a cross-machine direction is connected to a source of partial vacuum, and the core rotatably supports a perforate roll shell such that as the press felt extends around and wraps the pick-up roll shell, the roll shell rotates relative to the pick-up chamber. When the pick-up chamber is connected to the source of partial vacuum, the full-width sheet is pulled towards the press felt so that a transfer of the full-width sheet to the press felt is effected.
More recently, pick-up rolls have been proposed which include not only a pick-up chamber, but also a tail box or end box disposed adjacent to one edge of the web. The tail box, which is usually approximately six inches in width, is connected to a source of partial vacuum so that during a transfer operation, a tail cutter disposed upstream relative to the pick-up roll cuts a six inch wide tail from the formed web.
When the tail has been cut, both the tail and the remainder of the full-width sheet are discharged to a broke pit disposed beneath the fourdrinier wire.
The vacuum source is then connected to the tail box so that the tail is drawn towards the press felt by the vacuum in the tail box, and the tail is guided by the press felt through the press section.
Once the tail has been stabilized through the press section, the tail cutter is moved in a cross-machine direction across the forming wire such that the tail is widened to a full-width sheet.
In view of the movement of the formed web and the cross-machine directional movement of the tail cutter, the tail is widened and assumes a diagonal leading edge until the tail cutter completes movement across the web.
However, such prior tail threading operations have always been accomplished by turning off the vacuum supply to the tail box and by then turning on the vacuum supply to the cross-machine directional vacuum chamber during the tail widening operation.
Consequently, a problem has existed in that when the vacuum to the tail box is turned off, there exists a tendency for that edge of the web adjacent to the tail box to drop off of the press felt while the remainder of the tail, which is being widened to a full-width sheet, is being supported by the cross-machine directional vacuum chamber.
The present invention overcome the aforementioned problem by the provision of a downstream chamber which is connected to the same source of vacuum as the tail box so that during the tail widening step, a relatively low vacuum is applied along the entire cross-machine directional width of the widened tail, including that portion of the tail moving over the tail box.
When the tail has been widened to a full-width sheet, a relatively high vacuum is applied through an upstream vacuum chamber defined by the core for maintaining the transfer of the full-width sheet to the press felt.
Therefore, the present invention overcomes the aforementioned inadequacies of the prior art arrangements and provides a pick-up roll apparatus which makes a considerable contribution to the art of transferring a tail of a web from a forming section to a press section.
Another object of the present invention is the provision of a pick-up roll apparatus which includes a downstream vacuum chamber which is connected to a tail box for controlling the transfer of a tail from a forming wire onto a press felt while the tail is being widened to a full-width sheet.
Other objects and advantages of the present invention will be readily apparent to those skilled in the art by a consideration of the detailed description contained hereinafter, taken in conjunction with the annexed drawings.